Now you understand that this lens will stay with me: It's ultra-sharp, reasonably long, bright and a macro!
To prove my point I'll post two other images captured with the Sigma 150/2.8. The first one links you through to the 12MPix 3MB full-res glory of the poppies. Just have a look at the fine hairs at the poppy-stems!

Poppies (@ f/5.6 1/500 sec):

Slug (@ f/5.6 1/500 sec):

This lens will permanently replace my Nikon 180/2.8D. And even the micro-Nikkor 105/2.8 VR is an endangered lens-species by now depending on any insights regarding VR-efficiency and focus-reliability And I still might shine some strong flash-light into the lens and report about flare and glare...

I stumbled upon your samples on flickr and followed the link to this thread. Also I've read here for quite a while, this answer gave me the final reason to register.

Ok, so what I like to point out is that the performance of the 180mm as demonstrated by you differs from my own experience. I've shot some sports events with it, mainly soccer, on my former D80. I happen to have had the 105mm Micro VR on my D80 as well, but never at the same time. Although the Micro is sharper at 2.8, the 180mm is much sharper than displayed in you samples.

However, when I read that you took the photos from a short (albeit not for a macro) distance, it somehow explains this variance. I myself noticed, that the AF tends to front-focus at closer distances. If you used the AF, than the performance is obviously due to a front focus error. The appearance of the blur supports this theory, I think.

Like him or not, but Mr. Rockwell noticed this a few years ago as well and I could confirm his findings:
"On my F100 there is a little AF error at closer than about 40 feet. AF is fine beyond 40 feet. However, if your subject is closer than 40 feet the camera tends to focus a little bit closer than it should, lowering sharpness when shooting at f/2.8."

This was never an issue for me as on a soccerfield, the players usually are acting at quite a distance.

Best regards,
Christian

PS: In case you still want to sell the 180mm, I would be happy to get a quote, including shipping within germany

In the comaprison I shot the Nikon 180/2.8 at around 7m distance giving a magnification of 1:40. This is below the 1:50 that many count as "practically infinity" for lens testing purposes. But as I said, discard the f/5.6 results, they are certainly botched in a way as f/4.0 is clearly better.

On your remark as to focus: I have had encounters of front-focus with other lenses too and I compensate for that in my D300, but only if the compensation works on all distances. So if the lens is sharp at infinity, I'd never dial in any micro-AF compensation for closer distances as this is totally unpractical in shooting situations. But you're right: What I measure in my reviews is not only the optical sharpness of the lens but also the quality of the AF. So you get an impression of the whole system.

As to me selling the 180mm, there is a strict rule aginst buying/selling at Camera Labs. So regard my remark just as a statement about my final decision between the Sigma 150/2.8 and the Nikon 180/2.8.

Hmmmm just had a look at the Photozone review of the 150/2.8 and although it performs well ('very good') on the Canon, the Nikon sample exhibits 'excellent' scores. Obviously, being a Nikonian, you are not in a position to offer a complete view on the Canon mount version, but would you say that the heavy centering issue with the Canon mount copy tested at Photozone can be blamed for the lower performance or do the Nikonians have it better? It is of course mentioned that the 8Mp sensor of the 350D could be limiting the performance but would this have such an impact?

Do you think this can be put down to Sigma's infamous quality control or could it be just the way is it?

Don't get me wrong, I'd be happy with a lens with 'very good' performance, but it's interesting nonetheless.

Mark

Mark, the optical performance of the Sigma shows absolutely no difference between a Nikon-, Canon-, or other mount! The only things that show up in the reviews at photozone are:
1. sample variations
2. differences from the body
So, yes there might be sample variations, as with many Sigma lenses before. So the general advice for you is: Buy at a shop where you can give the lens back without prohibitive restocking fee!

Mark, I trust Klaus Schroiff's reviews very much. What he does when he get's a decentered lens for a review is to pick the best corner (of all four) and show those results. With my own reviews I try to do it the same way: because you don't want the review to reflect the real behaviour of a mal-adjusted lens but the potential it has once well-adjusted.
The thing where I'm leaving my area of competence is this: It seems that with decentered lenses even the best corner is probably beyond the potential of the corners of a well-centred copy of the same lens. If this is true, it's extremely hard to recognize and measure the full potential of a decentered lens.
To me the Sigma 150/2.8 for Canon seems just an example for my theory as the figures for the test on the copy for Nikon are just so much better. And you can see that Klaus has different scales for MTF figures on Canon vs Nikon bodies to allow for the difference in body resolution. So it's more than that...

No insight from Klaus Schroiff on this issue yet, but I assume he's on holiday.
Another strange observation: the Sigma seems to produce brighter images than the Nikon. Have not yet scientifically tested this but it has peeked my curiosity

Got feedback from Klaus! He confirms my assertion that "with decentered lenses even the best corner is probably beyond the potential of the corners of a well-centred copy of the same lens".
There you have it!

I've been looking at macro lenses to go with the 50D, and from looking around the Sigma 150 f/2.8 macro seemed good, but this thread seals it.

One thing I haven't managed to answer clearly is how much a TC on it might affect usable quality? Anyone ever tried it with a TC? I know it will also depend on the specific TC used but I'm curious what sort of hit or potential is there.

I read that if you use the 1.4x TC you don't get extra magnification (Actually less) but you do with the 2x. Although a friend of mine has used the 2x from sigma, and he said it lowers the IQ quite obvious. Never saw any samples though..